I hear the same squeal after it rains the night before or if I wash the car in the afternoon the day before and I back out of the driveway the next morning, it's most likely rust from moisture in the night cool air. You scrub off that thin layer of rust first time you brake getting a squeal then it's done.
The most likely explanation--in view of the fact that the problem goes away for the rest of the day--is that you are simply hearing the effects of some surface rust that builds up on the rotors overnight, and is worn away after the first one or two brake applications.